1. Field of the Invention
This invention relates generally to rotatable transverse flux electrical machines. The present invention more specifically relates to transverse flux alternators and motors producing low cogging torque and ripple torque.
2. Description of the Related Art
Alternators and motors are used in a variety of machines and apparatuses to produce electricity from mechanical movements. They find applications for energy production and transportation, to name a few. Alternators and motors can use Transverse Flux Permanent Magnet (TFPM) technologies.
Transverse flux machines with permanent magnet excitation are known from the literature, such as the dissertation by Michael Bork, Entwicklung and Optimierung einer fertigungsgerechten Transversalfluβmaschine [Developing and Optimizing a Transverse Flux Machine to Meet Production Requirements], Dissertation 82, RWTH Aachen, Shaker Verlag Aachen, Germany, 1997, pages 8 ff. The circularly wound stator winding is surrounded by U-shaped soft iron cores (yokes), which are disposed in the direction of rotation at the spacing of twice the pole pitch. The open ends of these U-shaped cores are aimed at an air gap between the stator and rotor and form the poles of the stator. Facing them, permanent magnets and concentrators are disposed in such a way that the magnets and concentrators that face the poles of a stator core have the opposite polarity. To short-circuit the permanent magnets, which in the rotor rotation are intermittently located between the poles of the stator and have no ferromagnetic short circuit, short-circuit elements are disposed in the stator.
Put otherwise, transverse flux electrical machines include a circular stator and a circular rotor, which are separated by an air space called air gap, that allows a free rotation of the rotor with respect to the stator, and wherein the stator comprises soft iron cores, that direct the magnetic flux in a direction that is mainly perpendicular to the direction of rotation of the rotor. The stator of transverse flux electrical machines also comprises electrical conductors, defining a toroid coil, which is coiled in a direction that is parallel to the direction of rotation of the machine. In this type of machine, the rotor comprises a plurality of identical permanent magnet parts, which are disposed so as to create an alternated magnetic flux in the direction of the air gap. This magnetic flux goes through the air gap with a radial orientation and penetrates the soft iron cores of the stator, which directs this magnetic flux around the electrical conductors.
In the transverse flux electrical machine of the type comprising a rotor, which is made of a plurality of identical permanent magnet parts, and of magnetic flux concentrators, the permanent magnets are oriented in such a manner that their magnetization direction is parallel to the direction of rotation of the rotor. Magnetic flux concentrators are inserted between the permanent magnets and redirect the magnetic flux produced by the permanent magnets, radially towards the air gap.
The transverse flux electrical machine includes a stator, which comprises horseshoe shaped soft iron cores, which are oriented in such a manner that the magnetic flux that circulates inside these cores, is directed in a direction that is mainly perpendicular to the axis of rotation of the rotor.
The perpendicular orientation of the magnetic flux in the cores of the stator, with respect to the rotation direction, gives to transverse flux electrical machines a high ratio of mechanical torque per weight unit of the electrical machine. These TFPM alternators are also known to generate significant cogging torque and ripple torque.
Cogging torque of electrical machines is the torque due to the interaction between the permanent magnets of the rotor and the stator slots of a Permanent Magnet (PM) machine. It is also known as detent or ‘no-current’ torque having a variable reluctance function of the position. This torque is position dependent and its periodicity per revolution depends on the number of magnetic poles on the stator. Typically, the fundamental frequency of the torque is twice the standard torque of the alternator and, in theory, produces a zero energy balance (when losses are neglected). Cogging torque is an undesirable component for the operation of such an electrical machine. It is especially prominent at lower speeds, with the symptom of jerkiness. Cogging torque results in torque as well as speed ripple; however, at high speed the electrical machine moment of inertia can significantly filter out the effect of cogging torque.
The ripple torque is a variation of the torque in respect of the nominal torque and is generally stated in percentage. Typically, the fundamental frequency of the ripple torque is about three times the fundamental frequency of a single phase of the torque in a triphased electrical machine. Ripple torque is generally represented by an altered sinusoidal wave. The ripple torque in electrical machines is caused by many factors such as cogging torque, the interaction between the MMF and the air gap flux harmonics, or mechanical imbalances, e.g. eccentricity of the rotor. Ripple torque is defined as the percentage of the difference between the maximum torque Tmax and the minimum torque Tmin compared to the average torque Tavg:((Tmax−Tmin)/Tavg)×100  Equation 1
Ripple torque in electrical machines is generally undesirable, since it causes vibrations and noise, and might reduce the lifetime of the machine. Extensive ripple torque can require measures such changes to the machine geometry that might reduce the general performance of the machine.
Under load, there is an additional component contributing to the ripple torque in addition to the cogging torque: Ripple torque due to the interaction between the magneto motive force (MMF) and the air gap flux harmonics. This component can be influenced by changes to the geometry of the electrical machine.
A machine with a low cogging torque might have a high ripple torque whereas a machine with a high cogging torque might have a low ripple torque. The interaction between the MMF and air gap flux harmonics can compensate or increase the cogging torque or ripple torque in different cases. Cogging torque cannot be acted upon by a change in voltage or current.
It is therefore desirable to produce an electrical machine producing low vibrations, cogging torque and low ripple torque. It is furthermore desirable to provide an electrical machine that minimizes recourse to electrical adjustments to minimize vibrations, cogging torque and ripple torque. It is also desirable to provide an electrical machine that is economical to produce. Other deficiencies will become apparent to one skilled in the art to which the invention pertains in view of the following summary and detailed description with its appended figures.